Respiratory syncytial virus (RSV) has long been recognized as the major viral pathogen of the lower respiratory tract of infants. It has also been implicated in lower respiratory tract disease in adults, especially the elderly and the immunocompromised. RSV is a high priority for vaccine development but efforts to develop a vaccine have so far failed. The failure in developing a vaccine has led to renewed interest in the pathogenesis of disease and the immune mechanisms surrounding protection.
RSV was first identified as the agent that causes chimpanzee coryza in 1956 and was subsequently isolated from children with pulmonary disease. Today, according to World Health Organization (WHO) estimates, a third of the 12.2 million annual deaths in children under the age of five are due to acute infections of the lower respiratory tract. Streptococcus pneumoniae, Haemophilus influenzae, and RSV are the predominant pathogens causing these infections. Of these pathogens, RSV has been described as the single, most important cause of serious respiratory tract infections in infants and young children.
RSV epidemics are seasonal, although the virus likely persists within communities. Peak infection rates occur during cold seasons in temperate climates. The virus affects about 90% of infants and young children by the age two. Most often, infection occurs in infants between the ages of six weeks and six months, with the highest incidence in children under three months of age. Previous infection does not prevent repeated infections that are common in all age groups. For example, in a study in Houston, Tex., infection rates were 68.8 per 100 child-years in infancy, and 82.6 per 100 child-years in the second year of life. In a study in Sweden, antibodies to RSV were produced in 87% of children by age 18 months, and in virtually all children by age three.
RSV may also be the cause of up to 5% of community-acquired lower respiratory tract infections in adults. In the elderly, RSV infection can be especially serious, with up to 10% of hospitalized cases leading to death.
RSV most commonly causes an upper respiratory tract infection. Rhinitis, cough, and sometimes fever characterize such infections. Acute otitis media occurs in up to one third of children with RSV illness. Both RSV and bacterial pathogens have been isolated from the middle ears of children with RSV. RSV also causes croup but the most common serious manifestations of infection are bronchiolitis and pneumonia in children. Signs of upper-respiratory tract involvement commonly precede those of the lower respiratory tract (bronchiolitis and pneumonia) by a few days, and fever, when present, is usually low grade. Death can occur in around 1% of children hospitalized with RSV infection, with the greatest risk of serious complications of infection occurring in children (and adults) with compromised cardiac, pulmonary, or immune function. In adults, RSV can cause exacerbation of chronic obstructive lung disease and congestive heart failure.
The RSV genome comprises a single strand of negative sense RNA that is 15,222 nucleotides in length and yields eleven major proteins. (Falsey, A. R., and E. E. Walsh, 2000, Respiratory syncytial virus infection in adults, Clinical Microbiological Reviews 13:371-84.) Two of these proteins, the F (fusion) and G (attachment) glycoproteins, are the major surface proteins and the most important for inducing protective immunity. The SH (small hydrophobic) protein, the M (matrix) protein, and the M2 (22 kDa) protein are associated with the viral envelope but do not induce a protective immune response. The N (major nucleocapsid associated protein), P (phosphoprotein), and L (major polymerase protein) proteins are found associated with virion RNA. The two non-structural proteins, NS 1 and NS2, presumably participate in viral replication but are not present in infectious virions.
RSV infects through the upper respiratory tract (particularly the nasopharynx) and the eyes. The virus has an incubation period of about three to five days. Infections with RSV occur annually in the first few years of life. Thus, the protective immunological response is incomplete. Local secretory IgA is believed to contribute to resistance to infection in the upper respiratory tract. Protection of the lower respiratory tract is mediated partly by serum IgG. The F and G surface glycoproteins are the only RSV proteins known to induce protective neutralizing antibodies. The G glycoprotein appears to play a role in both induction of protective immunity and disease pathogenesis. For example, studies in mice have shown that the G glycoprotein primes for a Th2 CD4+ T cell response, characterized by production of IL-4, IL-5, IL-13 and pulmonary eosinophilia. Eosinophil recruitment and activation are promoted by several factors, such as IL-4 and IL-5. Pulmonary eosinophilia is associated with significant to severe lung pathology and is presumably, in part, mediated by RSV G glycoprotein-induced Th2 CD4+ cells. Expression of G glycoprotein during acute infection in mice has been associated with a modified innate immune response characterized by decreased Th1 cytokine expression (e.g., IL-2 and gamma interferon), altered chemokine mRNA expression (e.g., MIP-1 alpha, MIP-1 beta, MIP-2, IP-10, MCP-1), and decreased NK cell trafficking to the infected lung.
Human RSV strains have been classified into two major groups, A and B. The G glycoprotein has been shown to be the most divergent among RSV proteins. Variability of the RSV G glycoprotein between and within the two RSV groups is believed to be important to the ability of RSV to cause yearly outbreaks of disease. The G glycoprotein comprises 289-299 amino acids (depending on RSV strain), and has an intracellular, transmembrane, and highly glycosylated stalk structure of 90 kDa, as well as heparin-binding domains. The glycoprotein exists in secreted and membrane-bound forms.
Cellular immunity appears to play a prominent role in recovery from RSV infection. Thus, individuals with cellular immunodeficiency (inherited or acquired) have more severe and long-lasting RSV infections than normal individuals. After RSV infection, normal children show an RSV-specific lymphocyte proliferation, which suggests T cell stimulation. An RSV-specific cytotoxic T lymphocyte response has also been described and is likely to be important to recovery from illness. Both CD4 and CD8 T lymphocyte subsets are involved in terminating RSV replication during infection. The same cytotoxic T lymphocyte response may also exacerbate or augment the clinical disease associated with RSV infection. This hypothesis has been used to explain the more severe disease seen with the formalin-inactivated RSV vaccine tested in the early 1960s.
Successful methods of treating RSV infection are currently unavailable. Infection of the lower respiratory tract with RSV is a self-limiting condition in most cases. No definitive guidelines or criteria exist on how to treat or when to admit or discharge infants and children with the disease. Hypoxia, which can occur in association with RSV infection, can be treated with oxygen via a nasal cannula. Mechanical ventilation for children with respiratory failure, shock, or recurrent apnea can lower mortality. Some physicians prescribe steroids. However, several studies have shown that steroid therapy does not affect the clinical course of infants and children admitted to the hospital with bronchiolitis. Thus corticosteroids, alone or in combination with bronchodilators, may be useless in the management of bronchiolitis in otherwise healthy unventilated patients. In infants and children with underlying cardiopulmonary diseases, such as bronchopulmonary dysphasia and asthma, steroids have also been used.
Ribavirin, a guanosine analogue with antiviral activity, has been used to treat infants and children with RSV bronchiolitis since the mid 1980s, but many studies evaluating its use have shown conflicting results. In most centers, the use of ribavirin is now restricted to immunocompromised patients and to those who are severely ill.
The severity of RSV bronchiolitis has been associated with low serum retinol concentrations, but trials in hospitalized children with RSV bronchiolitis have shown that vitamin A supplementation provides no beneficial effect. Therapeutic trials of 1500 mg/kg intravenous RSV immune globulin or 100 mg/kg inhaled immune globulin for RSV lower-respiratory-tract infection have also failed to show substantial beneficial effects.
In developed countries, the treatment of RSV lower-respiratory-tract infection is generally limited to symptomatic therapy. Antiviral therapy is usually limited to life-threatening situations due to its high cost and to the lack of consensus on efficacy. In developing countries, oxygen is the main therapy (when available), and the only way to lower mortality is through prevention.
Vaccination against RSV is, therefore, the preferred method for reducing RSV-related morbidity. A formalin-inactivated RSV vaccine, tested in the 1960s, was found to be immunogenic, with high rates of seroconversion. However, vaccinated children were found to lack protection from subsequent RSV infection. Furthermore, RSV naive infants who received the formalin-inactivated RSV vaccine, and who were naturally infected with RSV later, developed more severe disease than did a control group immunized with a trivalent parainfluenza vaccine. This experience has necessitated a very cautious approach to testing non-live virus vaccines in RSV naive infants.
However, the formalin-inactivated RSV vaccine failed to cause enhanced disease in older, previously infected children. In addition, studies in BALB/c mice suggest that prior live virus infection predisposes an individual for a safe immune response to non-live vaccines. Consequently, non-live vaccines are being tested in older children and adults previously infected with RSV. Non-live vaccines tested thus far appear to be safe in older children and adults, but their efficacy is unknown. It is hoped that, with new immunologic tools, researchers will be able to understand the pathogenesis of enhanced disease and use this information to design non-live virus vaccines that will be safe in the RSV naive individual.
Efforts toward developing a vaccine for infants and young children are now focused on live virus vaccines. A number of candidate vaccines have been tested in humans, but none have been shown to be sufficiently safe to be considered a viable vaccine for administration to young children.
Therefore, there is a need for safe and effective vaccines against RSV, especially for infants and children. There is also a need for therapeutic agents and methods for treating RSV infection at all ages and in immunocompromised individuals. There is also a need for scientific methods to characterize the protective immune response to RSV so that the pathogenesis of the disease can be studied, and screening for therapeutic agents and vaccines can be facilitated. The present invention overcomes previous shortcomings in the art by providing methods and compositions effective for modulating or preventing RSV infection.